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The tunnel's construction would be a crucial part of allowing London and Brighton to be linked by a train line.

When a railway line from London to Brighton was first proposed in the 1830s no fewer than six routes were suggested, only two of which came through the Merstham Gap north of what is now Redhill.

The winning route was an unexpected victor at the end of a Parliamentary enquiry.

Even then, the line should have gone through or near Reigate rather than two miles to the east of it. According to one account, opposition from local landowners prevented it doing so, but it is more likely that the topography of the area was the cause.

The route chosen followed that of the new Brighton Road, opened in 1818 through the gap between Redstone Hill and Redhill Common.

At that time Redhill as a town did not exist. The area now occupied by the town centre was empty marshland devoid of any buildings. There were a few farms in the vicinity and a cluster of cottages but that was about it.

When the tunnel was finished, to make the public feel safe in the darkness, gas lamps were fitted to the walls which were whitewashed.

This was soon abandoned, however, after the large amount of soot emitted from the trains made it too difficult to keep bright.

Trains between London and Brighton pass through the tunnel (Photo by Dan Kitwood/Getty Images)

The tunnel was something of an engineering marvel but in 1905 it gained notoriety for a far darker reason when it became the site of a murder mystery.

On September 24, 1905, a 22-year-old woman's body was found mutilated inside the tunnel by a sub inspector, William Peacock.

Peacock found Mary Sophia Money shortly before 11pm, while her body was still warm, and took her to the nearest train station where police instructed him to bring it to The Feathers Inn.

Mary, a bookkeeper, did not have any identifying papers on her and the day after her brother, Robert Henry Money, a dairy farmer, had to identify her.

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This theory was further strengthened after a guard at Purley Oaks Signal Box and a guard at East Croydon recalled seeing a couple fighting.

Both guards said the woman in the couple fitted Mary's description.

Following her murder, over 100 interviews were taken and a huge investigation was sparked.

But the killer could never be found and her unsolved murder is widely regarded as the first murder on a train in the country.

Only half the story

You would probably have to be very observant to notice but there are actually two tunnels, almost side by side, which trains run through between Coulsdon and Merstham.

The sharing of the main line caused a great deal of friction between rail operators South Eastern Railway (SER) and the London Brighton and South Coast Railway (LB&SCR).

Eventually the LB&SCR gained Parliamentary approval to build its own independent line between Coulsdon North and Earlswood, which bypassed the SER stations of Coulsdon South, Merstham and Redhill.

A train on the Quarry Line

This involved the construction of a second tunnel to the east of the original, and 25 feet (7.6 metres) above the level of the original. Since both routes form part of the Brighton Main Line, in order to differentiate them the former was called the "Redhill Line", whilst the new line became known as the "Quarry Line".

The newer tunnel, known as the Quarry Tunnel, is about 1.2 miles long and was built between 1896 and its opening on November 8, 1899.

Today fast trains that bypass Redhill and Mertsham will still use the Quarry Tunnel while trains that stop at Redhill and Mertsham use Merstham Tunnel.